The present invention relates to polishing pads for chemical mechanical polishing (CMP), and in particular relates to methods of forming layered polishing pads, including such pads having windows.
Semiconductor wafers having integrated circuits fabricated thereon must be polished to provide a very smooth and flat wafer surface which in some cases may vary from a given plane by as little as a fraction of a micron. Such polishing is usually accomplished in a chemical-mechanical polishing (CMP) operation, which utilizes a chemically active slurry that is buffed against the wafer surface by a polishing pad.
One problem associated with the CMP operation is pad performance. Certain polishing pads are made up of layers of material. Over time, due the wear and tear of the polishing process, the bonds between the different material layers can weaken. This can result in delamination of the layered pad.
Another problem associated with the CMP operation is determining when the wafer polishing process should stop. Conventional methods for determining a polishing endpoint require that the polishing operation be stopped and that the wafer be removed from the polishing apparatus so that dimensional characteristics can be determined. However, stopping the CMP operation slows down wafer production. Further, if a critical wafer dimension is found to be below a prescribed minimum, the wafer may be unusable, thereby leading to higher scrap rates and production costs.
In-process methods for determining polishing endpoint have also been developed. For example, U.S. Pat. No. 5,413,941 discloses an optical inspection method that utilizes laser interferometry to measure a wafer dimension.
Polishing pads with windows have been developed to facilitate optical inspection methods during CMP. For example, U.S. Pat. No. 5,605,760 discloses a polishing pad that includes a transparent window. The window is a rod or plug of transparent polymer material. The rod or plug is either insert molded within the polishing pad, or is installed into a cutout in the polishing pad after the molding operation.
The prior art polishing pads with windows have a number of disadvantages. For instance, a number of manufacturing steps are required to either install a window piece into a hole in the pad, or into the mold cavity in which the pad is produced. In some cases, a hole to receive the window piece must be cut into the pad. Also, leakage of slurry between the pad and the window is often a problem.
U.S. Pat. No. 6,524,164 (the '164 patent) discloses a polishing pad having a transparent window and that is designed to mitigate leakage. The polishing pad includes an impermeable transparent sheet between the polishing pad and the subpad to prevent slurry leakage. To fabricate the pad, the transparent sheet needs to have an adhesive layer on each side, and the sheet needs to be laminated to the lower surface of the polishing pad. This leaves a double-sided adhesive layer on the lower side of the pad window.
While the pad of the '164 patent initially performs well, the transparency of the adhesive layer on the window degrades over time. This can reduce the effectiveness of the optical inspection processes.